1. Field of the Invention
The present invention relates to tightening tools constructed to tighten a fastener such as a bolt, a nut, or a screw, and more particularly relates to tightening tools provided with a clutch that limits the torque transmitted from the motor to the tool bit.
2. Description of the Related Art
Publication of Japanese Examined Utility Model Application S50-33759 sets forth a tightening tool for tightening a fastener with a specified torque. This tightening tool comprises a clutch that limits the torque transmitted from the motor to the tool bit. The clutch comprises a pair of clutch members, and a spring member that pushes one of the clutch members towards the other. One clutch member of the pair is connected at a motor side, and the other clutch member of the pair is connected at a tool bit side. In this clutch, the torque is transmitted from the motor to the tool bit by means of mutually engaging the pair of clutch members, and the torque transmission from the motor to the tool bit is halted by disengaging the clutch members.
Engaging portions are formed on the pair of clutch members. The engaging portions are constructed so as to maintain engagement between the pair of clutch members while the torque applied to the tool bit (this is equivalent to the torque that the tool bit applies to the fastener) is less than a predetermined value, and are constructed so as to disengage the pair of clutch members when the torque applied to the tool bit reaches the predetermined value. The transmission of torque from the motor to the tool bit is consequently halted in this tightening tool at the point when the tightening torque of the fastener reaches the predetermined value.
The amount of torque required to cause disengagement of the clutch members varies according to the shape of the engaging portions and the pushing force applied by the spring member. Generally, the tightening tool is constructed in such a way that the pushing force applied from the spring member can be adjusted, and thus the torque that causes clutch members to disengage can be adjusted. The torque transmitted from the motor to the tool bit can thus be limited to the tightening torque specified for the fastener.